1. Technical Field
The present disclosure relates to a method of adjustment during manufacture of a capacitor laid on a substrate and, by way of example, the present disclosure relates to a method of manufacturing that provides for adjusting the frequency of a circuit having a resonant element, for example, of a monolithic oscillator using bulk acoustic wave (BAW) resonators.
2. Description of the Related Art
Oscillators are mainly used in electronic devices to provide clock signals at reference frequencies. Currently, an oscillator includes oscillating circuit elements and a quartz resonator that enables accurately setting the oscillation frequency.
An alternative to quartz oscillators is the use of oscillators based on BAW resonators. The use of BAW resonators enables implementing higher oscillation frequencies, for example, approximately ranging from a few hundreds of MHz to a few tens of GHz. Lower clock frequencies may also be generated by using, at the oscillator output, a frequency-division circuit. Further, BAW resonators have reduced footprint and offer the advantage of having a good quality factor.
It has also been provided to form a monolithic oscillator using a BAW resonator, that is, an oscillator in which the oscillating circuit elements and the resonator are assembled inside and on top of a same integrated circuit chip. The oscillating circuit elements may be formed inside and on top of a semiconductor substrate, for example, a silicon wafer. The BAW resonator is then deposited above this substrate and connected to the oscillating circuit elements. Such an oscillator has the advantages of being very compact and of providing good performance.
FIG. 1 is a cross-section view schematically showing a BAW resonator 1 formed on a semiconductor substrate 3. In this example, the substrate is coated with an insulator 4. Resonator 1 includes a resonator core or piezoelectric resonator 5 formed of two electrodes 5a, 5c between which is arranged a layer 5b of a piezoelectric material. When an electric field is created in the piezoelectric layer by application of a potential difference between electrodes, this results in a mechanical disturbance in the form of acoustic waves. The waves propagate in the resonator. The fundamental resonance establishes when the acoustic wavelength in the piezoelectric material substantially corresponds to twice the thickness of piezoelectric layer 5b. 
An acoustic isolation device is provided between the resonator core and the substrate to avoid losing acoustic waves in the substrate. There mainly exist two types of BAW resonators: BAW resonators deposited on a membrane, and BAW resonators mounted on the substrate.
BAW resonators deposited on a membrane, such as the resonator 1, such as FBARs (Film Bulk Acoustic Wave Resonators) or TFRs (Thin Film Resonators) form a recess 7 between the resonator core and the substrate. A membrane 8 supports the various layers of the resonator above the recess 7.
BAW resonators mounted on the substrate, or SMRs (Solidly Mounted Resonators), are generally isolated from the substrate by an acoustic reflector, currently a Bragg mirror.
FIG. 2 shows a simplified electric diagram of an oscillator with a BAW resonator 25. This oscillator has various elements of a circuit 23, connected between a high voltage power supply terminal VCC and a terminal of low voltage, for example, the ground, and the BAW resonator 25, connected to circuit elements 23.
The circuit 23 especially includes active elements capable of sustaining oscillations and of amplifying an output signal OUT, and passive elements, for example, capacitors. The BAW resonator 25 enables to select the oscillation frequency.
FIG. 3 shows the circuit of FIG. 2 in a more detailed fashion in which the circuit 23 is a Colpitts oscillator. In this example, the circuit 23 more particularly includes a MOS transistor 31 series-connected with a current source 33 between a high supply voltage terminal VCC and the ground. Two capacitors 35 and 37 are series-connected between the gate of transistor 31 and the ground. A resistor 39 is connected between high voltage power supply terminal VCC and the gate of the transistor 31. The terminal or node common to the capacitors 35 and 37 is connected to the drain of the transistor 31. The BAW resonator 25 is connected between the gate of the transistor 31 and the ground. The oscillator output is connected to the source of the transistor 31.
The transistor 31 and the current source 33 amplify the output signal and sustain the oscillations. The frequency of output signal OUT is especially dependent on the resonance frequency of the resonator 25 and on the capacitances of the capacitors 35 and 37.
In practice, it is difficult in manufacturing to obtain an oscillation frequency with the desired accuracy.
A first source of inaccuracy is due to the BAW resonator manufacturing dispersions. Indeed, methods of deposition of the different layers of a BAW resonator do not enable obtaining a resonance frequency with the desired accuracy. Substantial variations of the resonance frequency can especially be observed between resonators formed from a same substrate wafer.
For this reason, as illustrated in FIG. 1, a frequency adjustment layer 9, for example made of silicon nitride, is provided at the surface of the resonator 1. The presence of this layer modifies the behavior of the resonator, and especially its resonance frequency. In a manufacturing step, the layer 9 is thinned down by local etching, for example, by ion etching, to get closer to the aimed resonance frequency.
Despite this adjustment, the accuracy of the BAW resonators is not ideal.
A second source of inaccuracy results from manufacturing discrepancies in the elements of circuit 23. Indeed, despite the attention brought to the forming of these elements, behavior differences can be observed between circuits formed inside and on top of a same substrate wafer.
To overcome this lack of accuracy, a variable capacitance, for example a network of switched capacitors, is generally used in the circuit 23, at least for a portion of one of the two capacitors 35 and 37. The frequency of the output signal of each oscillator can thus be finely corrected in a final calibration step when the BAW resonator is connected to the circuit 23 and the oscillator is powered.
A disadvantage of this calibration mode is that, to be able to compensate for the above-mentioned significant inaccuracy of the oscillation frequency, a large network of switched capacitors must be provided.